About 4.6 billion years ago, a large molecular cloud gave birth to the Sun. Within our Solar System, the Sun contains 99.8% of its mass, the rest going to the planets, moons, comets, and asteroids. The Sun is now halfway through its expected lifetime. During the course of a human lifetime, the Sun does not change much. It rises and sets the same times each year, its energy output does not vary much, and the average human will see about seven solar cycles. Over the course of billions of years, the Sun does and will continue to evolve. If the human race survives that long, that will have implications for its future.
The majority of the Sun’s life is spent on what astronomers call the main sequence. During this time, the Sun fuses hydrogen into helium, a fraction of this mass is converted into energy providing the sustenance for life on Earth. This reaction converts 4 hydrogen atoms into 1 helium atom plus two left over hydrogen atoms. The process converts 0.71% of the original 4 hydrogen atoms’ mass into energy. Each second, the Sun transforms 4 million tons of mass into energy. If the Sun was the size of Earth, this would be the equivalent of converting 12 tons of mass into energy each second. You might worry that this would burn up the Sun in short order, but the Sun is very large and if you divide its mass by 4 million, it would use up its mass in 4.9725 × 1023 seconds, or 1.58 × 1016 years. The Sun will not exist that long as there are other factors in play.
There are two major forces acting within the Sun. One is the force of gravity as the Sun’s mass compresses its core. This compression heats up the core to a temperature of 15 million K. A temperature of 12 million K is required to start nuclear fusion. Here you see the challenge of using fusion as an energy source on Earth. Hydrogen bombs use fusion to explode, but require a fission atomic bomb to detonate it by delivering the required heat to start the fusion process. Controlled fusion would make for a great energy source on Earth, but it is problematic to create a temperature of 12 million K. Current research is looking into high energy lasers to heat hydrogen enough to commence controlled fusion.
Once fusion starts in the Sun’s core, this creates the second force in play, an outward pressure generated by heat. This outward force perfectly balances the inward force of gravity preventing the Sun from collapsing upon itself. This balancing act, referred to as hydrostatic equilibrium, is one of nature’s great regulators. It is this balancing act that regulates short-term solar output so that it varies only a fraction of a percent. This modulation of solar output provides a stable environment on Earth required for life. However, over the course of a few billion years, it’s a different story.
As the Sun’s core converts hydrogen into helium, it becomes denser and hotter. This in turn gradually makes the Sun more luminous. The Sun is 30% more luminous today than 4 billion years ago. In about 1 billion years, the Sun will become hot enough to boil off the oceans on Earth. If humanity can survive its foibles over that time, it will need to move off the Earth to exist. Colonizing Mars within that time frame is certainly doable. What may not be doable, is interstellar colonizing when the Sun ends its main sequence stage. Just before that occurs, another event will impact the Sun.
In about 4 billion years, the Milky Way will collide with its neighbor, the Andromeda galaxy. While galaxies frequently collide, stars do not. If the Sun was the size of a grain of sand, the nearest star would be another grain of sand over four miles away. What could happen is the Sun may be ejected from the Milky Way. The result of this collision is that the two spiral galaxies will combine to form one giant elliptical galaxy in a process that will cover 2 billion years (video below). It’s impossible to model whether or not the Sun will be part of this new galaxy, but either way, the Sun will become a red giant afterwards.
A star becomes a red giant when it runs out of hydrogen in its core. The rate of fusion slows down causing gravity to compress the core. As a result, the shell of hydrogen outside the now helium core ignites. The hotter core creates an outward pressure expanding the star greatly. When the Sun turns into a red giant in 5 billion years, Mercury, Venus, and possibly Earth will be incinerated. A red giant’s surface is much cooler than the Sun is today, but is much more luminous. That may sound counter-intuitive, but think of it this way. One 100-watt light bulb is brighter than one 60-watt light bulb. However, 100 60-watt light bulbs is brighter than one 100-watt light bulb. Besides temperature, stellar radius also factors into a star’s luminosity. The Sun still has a few more steps to complete in its life cycle.
The red giant phase of the Sun will end in a helium flash. This occurs when the core is compressed to a degenerate state where electrons are packed to the point where all possible states are occupied. The compression heats the core to the required 100 million K to commence helium fusion into carbon. This in turn breaks down the degenerate state of the core and the Sun will become a yellow giant. The Sun is not large enough to fuse carbon.
However, the intense heat of helium fusion will generate even more outward pressure and expand the Sun’s radius even further so its outer shell becomes transparent, and cool. So cool, that elements such as carbon and silicon solidify into grains and are expelled out by an intense solar wind. At this stage, the Sun will be a Mira variable for 10 million years. After this, the Sun will enter the final stages of its life as a white dwarf surrounded by a planetary nebula.
A white dwarf is the exposed core of a star. Comprised of carbon and oxygen, it is not large enough to fuse atoms. Its heat is akin to a car engine still being warm after it has been turned off. While an engine will cool off in a few hours, it will take trillions of years for a white dwarf to go completely dark. This is longer than the current age of the universe at 13.7 billion years. The planetary nebula’s life is much shorter.
The term planetary nebula is a holdover from the days when these nebulae resembled planets in telescopes. With the Hubble Space Telescope, we now know planetary nebulae can also take the shape of bipolar jets. How the Sun will look we do not know. We do know that the core will no longer be capable of holding on to its outer shell. The planetary nebula will disperse into interstellar space in 10,000 years.
These gases will not only hold the remnants of the Sun, but the planets and the very atoms that make up our bodies. The Sun itself is a remnant of a prior star. We know this as trace amounts of metal exist in the Sun. These metals are produced by fusion, or if the star is large enough, a supernova explosion. Colliding galaxies compress interstellar gas igniting star formation. As the Andromeda galaxy collides with the Milky Way, it is very possible what used to make up the Sun will form a new star, with planets, and possibly, plants, then animals, and finally, intelligent beings.
The cycle of life begins anew.
*Image atop post is from NASA’s Solar Dynamics Observatory.